Caption: Blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes, red) and two white blood cells (leucocytes, cream). Red blood cells are carriers of oxygen and carbon dioxide. Their biconcave surface provides maximum surface area for the exchange of gases. There are a number of different types of white blood cell, all of which are part of the immune system. Magnification: x4400 when printed 10 centimetres wide.